1. Field of the Invention
The present invention relates, generally, to torsional dampers and, more specifically, to a variable torsional damper having magneto-rheological fluid damping in parallel to a spring damper.
2. Description of the Related Art
In automotive applications, engine torque and speed are translated between a prime mover, such as an internal combustion engine, to one or more wheels through the transmission in accordance with the tractive power demand of the vehicle. Torsional damping mechanisms are well known in the related art for reducing vibrations and torque pulses between the prime mover and the transmission. In a manual transmission for example, one torsional damping mechanism commonly known in the art includes a plurality of coiled springs mounted in a clutch disk between the input member and the output member that is further connected to the input shaft of the transmission. This arrangement acts to dampen torsional vibrations due to impact loads and pulsations generated between the prime mover and the transmission. Torsional dampers of this type may also be employed between the prime mover and a start-up clutch or a pair of flywheels associated with the prime mover and the transmission, respectively in other types of torque transmitting devices.
Torsional dampers of these known types are generally referred to as fixed hysteresis dampers where the torsional damping depends on the fixed compression rates of the springs. While conventional torsional dampers having fixed compression rates of the type employed in the related art have generally worked for their intended purposes, they are known to suffer from certain disadvantages. For example, it is not uncommon that, during vehicle launch, the torsional damper is subjected to relatively high torque peaks. When this occurs, it is possible for the coiled springs to be over-compressed to the point that they “bottom out.” When this occurs, the relative rotation between the drive and driven members is described as “over-travel” and results in the generation of noise and vibration through the vehicle driveline. Over-travel is a condition of “high hysteresis” between the drive and driven members. Over-travel may be combated by employing stiffer coiled springs. However, with the increase in the stiffness of the coiled spring, there is an associated decrease in damping through the torsional damper. On the other hand, following vehicle launch and at high rotational speeds, the input and output members of the torsional damper rotate, for the most part, substantially together so that there is little or no relative rotation therebetween. Thus, the torsional damper operates in a condition of “low hysteresis.” In this operative mode, the coiled springs adequately function to absorb the minimal torque pulses and vibrations that may be generated between the prime mover and the transmission.
There have been a number of solutions that have been proposed in the related art to address the problems associated with high hysteresis during launch and a low hysteresis during higher rotational speeds after launch. However, the conventionally known torsional dampers that embody these solutions typically employ a relatively high number of components and an associated increase in cost to address the operational challenges that are placed on the torsional dampers. Likewise, developments in creating variable torsional dampers, where the hysteresis is variable based on varying load conditions also embody many components that add a great deal of weight. The complexities of known approaches to variable torsional dampers also make for prohibitively expensive devices. However, the dynamic damping of variable torsional dampers has proven to be much more effective than fixed torsional dampers. Thus, there is a need in the art for a variable torsional damper that effectively dampens impact loads, pulsations, torque peaks, and vibrations between the prime mover and the transmission, while remaining relatively mechanically simple such that prohibitive weight or cost in producing a variable torsional damper is avoided.